Tepco on Friday started increasing the amount of water it is injecting into the No. 1 reactor at the Fukushima No. 1 power plant in hopes of providing stable cooling for the damaged nuclear fuel inside.

Tokyo Electric Power Co. plans to fill the reactor’s primary containment vessel with enough water to submerge the fuel over the next 20 days or so, and start operating by June an air-cooling device it hopes will reduce the temperature of the water circulating around the reactor, company officials said.

Restoring the reactors’ cooling systems, which were lost in the March 11 quake and tsunami, is vital to ending the nuclear crisis because its current emergency cooling measure, which requires that water be continually injected from outside, has created vast pools of highly radioactive water within the plant.

Tepco’s goal is to install stable cooling systems for reactors 1 through 4 and their spent-fuel pools in about three months, according to a crisis plan it announced April 17.

On Friday, it increased the amount of water injected into the pressure vessel housing the core of the No. 1 reactor to 8 tons per hour from 6 tons. Its goal is to fill the primary containment vessel with about 7,400 tons of water.

The reactor fuel is inside a pressure vessel that is further contained in a flask-shaped primary containment vessel. Each reactor building houses a vessel.

Before increasing the water volume, Tepco on Thursday told the Nuclear and Industrial Safety Agency that the container, even when flooded with such a large amount of water, can withstand any aftershock from the magnitude 9 earthquake that struck on March 11.

The agency endorsed the report as appropriate but ordered Tepco to consider reinforcing part of the container to enhance quake safety, since it was not designed to be filled up.

Work to reduce the high radiation inside the No. 1 reactor building is also proceeding so that engineers will be able to work more efficiently to set up the equipment needed to install the new cooling system.